Method for producing carboxylic acid esters

ABSTRACT

The invention relates to a method for producing carboxylic esters by converting a carboxylic acid or a carboxylic acid anhydride or a mixture thereof with an alcohol in a reaction system comprising one or more reactors, wherein reaction water is distilled as alcohol-water-azeotrope with the vapors, the vapors are at least partially condensed, the condensate is separated into an aqueous phase and an organic phase and said organic phase is supplied at least partially back into said reaction system. Components boiling lower than the alcohol are at least partially removed from said returned organic phase such as wherein components boiling lower than alcohol are evaporated and/or distilled off. An enrichment in the reaction system of by-products boiling lower than alcohol is avoided. Alcohol losses can be minimized by discharge currents.

The invention relates to a process for preparing carboxylic esters byreacting carboxylic acid or a carboxylic anhydride or a mixture thereofwith an alcohol.

Esters of phthalic acid, adipic acid, sebacic acid or maleic acid arewidely employed in surface coating resins, as constituents of paints andin particular as plasticizers for plastics.

It is known that carboxylic esters can be prepared by reactingcarboxylic acids with alcohols. This reaction can be carried outautocatalytically or catalytically, for example in the presence ofBrönsted or Lewis acids as catalysts. Regardless of the type ofcatalysis, there is always a temperature-dependent equilibrium betweenthe starting materials (carboxylic acid and alcohol) and the products(esters and water).

The reaction of internal carboxylic anhydrides with alcohols proceeds intwo steps: the alcoholysis of the anhydride to form the monoestergenerally proceeds rapidly and to completion. The further conversion ofthe monoester into the diester with formation of water of reaction isreversible and proceeds slowly. This second step is the rate-determiningstep of the reaction.

To shift the equilibrium in the direction of the ester (or the fullester in the case of polybasic acids), it is usual to use an entrainerby means of which the water of reaction is removed from the mixture. Ifone of the starting materials (alcohol or carboxylic acid) has a boilingpoint lower than that of the ester formed and forms a miscibility gapwith water, a starting material can be used as entrainer and berecirculated to the mixture after water has been separated off. In theesterification of higher aliphatic carboxylic acids, aromatic carboxylicacids or dibasic or polybasic carboxylic acids, the alcohol used isgenerally the entrainer.

EP-A 1 186 593 describes a process for preparing carboxylic esters byreacting dicarboxylic or polycarboxylic acids or anhydrides thereof withalcohols, with the water of reaction being removed by azeotropicdistillation with the alcohol. The amount of liquid removed from thereaction by the azeotropic distillation is replaced either completely orpartly by the alcohol.

If the alcohol used serves as entrainer, it is usual to condense atleast part of the vapor from the reactor, separate the condensate intoan aqueous phase and an organic phase comprising essentially the alcoholused for the esterification and recirculate at least part of the organicphase to the reactor. However, various by-products are formed inaddition to the desired ester in the esterification reaction.Particularly the by-products having lower boiling points than that ofthe alcohol are recirculated with the organic phase to the reactor andcan, particularly in the case of a continuous process, accumulate in thereaction system.

It has been found that the by-products having boiling points lower thanthat of the alcohol comprise mainly olefins which have been formed byelimination of water from the alcohol used. Relatively highconcentrations of the olefins can damage the esterification catalystused and/or impair the quality of the ester product produced, inparticular lead to undesirable discoloration. To avoid accumulation ofthe by-products having boiling points lower than that of the alcohol inthe reaction system, it is advisable not to recirculate all of theorganic phase to the reactor but rather to discharge a substream (purgestream). However, a considerable proportion of the alcohol used is lostto the esterification reaction via the purge stream.

It is therefore an object of the invention to minimize the alcohollosses via the purge stream.

The present invention accordingly provides a process for preparingcarboxylic esters by reacting a carboxylic acid or a carboxylicanhydride or a mixture thereof with an alcohol in a reaction systemcomprising one or more reactors, with water of reaction being distilledoff as alcohol-water azeotrope with the vapor, the vapor being at leastpartly condensed, the condensate being separated into an aqueous phaseand an organic phase and at least part of the organic phase beingrecirculated to the reaction system, wherein components having boilingpoints lower than that of the alcohol (hereinafter also: “low boilers”)are at least partly removed from the organic phase to be recirculated.

For the purposes of the present invention, a “reaction system” is areactor or an assembly of a plurality of reactors. In the case of aplurality of reactors, these are preferably connected in series.“Recirculation to the reaction system” means that an organic phase isintroduced into at least one reactor which may be chosen freely of thereaction system. The process of the invention can be carried outbatchwise or continuously, but is preferably carried out continuously.The low boilers comprise or consist essentially of olefins (generallyolefin isomer mixtures) which are derived from the alcohol used byelimination of water.

The reactors can be any reactors which are suitable for carrying outchemical reactions in the liquid phase.

Suitable reactors are reactors which are not backmixed, e.g. tubereactors or residence vessels provided with internals, but preferablybackmixed reactors such as stirred vessels, loop reactors, jet loopreactors or jet nozzle reactors. However, combinations of successivebackmixed reactors and reactors which are not backmixed can also beused.

If appropriate, a plurality of reactors can also be combined in amultistage apparatus. Such reactors are, for example, loop reactors withbuilt-in sieve trays, cascaded vessels, tube reactors with intermediatefeed points or stirred columns.

In a further process variant, the reaction can be carried out in areactive distillation column. Such columns have a long residence time ofthe reaction solution in the respective stage. Thus, for example,columns which have a high liquid hold-up, e.g. highly banked-up trays ofa tray column, can advantageously be used.

Preference is given to using stirred tank reactors. The stirred tankreactors are usually made of metallic materials, with stainless steelbeing preferred. The reaction mixture is preferably intensively mixed bymeans of a stirrer or a circulation pump.

Even though the process of the invention can be carried out using onlyone reactor, it is nevertheless advantageous to connect a plurality ofreactors, e.g. stirred vessels, to one another in the form of a reactorcascade in order to obtain a substantially complete reaction. Thereaction mixture passes through the individual reactors in succession,with the discharge from the first reactor being fed to the secondreactor, the discharge from the second reactor being fed to the thirdreactor, etc. The reactor cascade can comprise, for example, from 2 to10 stages, with from 4 to 6 stages being preferred.

During the reaction, an alcohol/water mixture is distilled off asazeotrope from the reaction mixture. In addition, further alcohol is fedinto the reactor or the individual reactors of the reaction systemduring the reaction. It is advantageous to feed alcohol into therespective reactor at a predetermined flow rate. The flow rate can beadapted as a function of the periodically measured acid number of thereaction mixture in the respective reactor.

The condensation or partial condensation of the vapor can be effectedusing all suitable condensers. These can be cooled by means of anycooling media. Condensers having air cooling and/or water cooling arepreferred, and air cooling is particularly preferred.

The condensate obtained is subjected to a phase separation into anaqueous phase and an organic phase. For this purpose, the condensate isusually introduced into a phase separator (decanter) where it separatesby mechanical settling into two phases which can be taken offseparately. The aqueous phase is separated off and can, if appropriateafter work-up, be discarded or used as stripping water in theafter-treatment of the ester.

The vapor from the individual stirred vessels of a cascade can becombined and subject together to the removal of low boilers according tothe invention. It has been found that even only one condenser and oneapparatus for separating off low boilers can be sufficient to achieveeffective treatment of the vapors coming from a plurality of vessels ofa cascade.

If appropriate, a plurality of vessels of the cascade can be combined toform one subunit, with the subunits then each being coupled to acondenser and an apparatus for separating off low boilers. It is alsopossible to couple each vessel of the cascade with a condenser.

The organic phase treated according to the invention and to berecirculated can be passed into any reactor of a cascade or distributedover a plurality of reactors of the cascade. However, the organic phasetreated according to the invention and to be recirculated is preferablynot introduced into the last reactor of the cascade. The organic phasetreated according to the invention and to be recirculated is preferablyintroduced exclusively or predominantly into the first reactor of thecascade.

There are various possibilities for the recirculation of the organicphase into the reaction system. One possibility is to pump the organicphase, if appropriate after heating, into the liquid reaction mixture.

However, to thermally optimize the process, the organic phase ispreferably recirculated into the reaction system via a column (known asrecycle alcohol column) in which the organic phase recirculated isconveyed in countercurrent to at least part of the vapor. The organicphase is advantageously introduced into the recycle alcohol column atthe top or in the upper region. The descending condensate of the recyclealcohol column goes back into the reaction system, when a reactorcascade is used preferably into the first reactor. The recirculation ofthe organic phase via the recycle alcohol column has the advantage thatthe recirculated organic phase is preheated and freed of traces of waterwhich have remained in the organic phase after the phase separation orare, in accordance with their thermodynamic solubility, dissolved in theorganic phase. The recycle alcohol column can be, for example, a traycolumn, a column having ordered packing or a column having randompacking elements. A small number of theoretical plates is generallysufficient. A column having, for example, from 2 to 10 theoreticalplates is suitable.

When a reactor cascade is used, the vapor preferably leaves at least thefirst reactor via the recycle alcohol column. One or more or all furtherreactors can likewise have a vapor offtake to the recycle alcoholcolumn.

According to the invention, components having boiling points lower thanthat of the alcohol are at least partly removed from the organic phaseto be recirculated. Here, the boiling point differences betweenolefin/alcohol and olefin-water azeotrope/alcohol-water azeotrope areexploited. The order of boiling points is illustrated below for theexample of 1-nonene/1-nonanol and their azeotropes with water:

Boiling point [° C.] 1-Nonene-water 94.327 minimum heteroazeotrope1-Nonanole-water 99.719 minimum heteroazeotrope 1-Nonene 146.9031-Nonanol 213.396

In one embodiment of the process, the vapor comprising the alcohol-waterazeotrope is condensed incompletely, resulting in components havingboiling points lower than that of the alcohol accumulating in theuncondensed vapor and being able to be discharged with the uncondensedvapor. The condensate is separated into an aqueous phase and an organicphase comprising essentially alcohol and the organic phase is at leastpartly recirculated to the reaction system.

Incomplete condensation of the vapor can be achieved by appropriateselection of the temperature in the condenser, e.g. by choice of thetemperature and/or flow of the cooling medium. To effect incompletecondensation of the vapor, the latter can also be introduced e.g. asbottom or side feed stream, into a column. The uncondensed vapor can becondensed in an after-condenser and, for example, passed to thermalutilization.

In this embodiment, the separation of alcohol and low boilers occurs inthe presence of water; the separation is based on the different boilingpoints of the alcohol-water azeotrope and the olefin-boiler azeotrope.As is shown in the table above, the boiling point difference between theazeotropes is not pronounced, so that only incomplete separation ispossible at a small number of theoretical plates. In addition, formationof complex mixtures which comprise not only the alcohol-water azeotropeand the olefin-water azeotrope but also alcohol, olefin, etc., canoccur. Since the boiling point of the alcohol-water azeotrope isgenerally lower than the boiling point of the olefin, only incompleteremoval of the low boilers is achieved in this embodiment. Theuncondensed vapor therefore still comprises a large proportion ofalcohol which is lost to the esterification reaction.

In another, preferred embodiment of the process, the vapor comprisingalcohol-water azeotrope is therefore at least partly condensed, inparticular essentially completely condensed, and the condensate isseparated into an aqueous phase and an organic phase. At least part ofthe organic phase is treated by evaporating and/or distilling offcomponents having boiling points lower than that of the alcohol andrecirculating at least part of the organic phase which has been treatedin this way to the reaction system. The low boilers which have beenevaporated or distilled off can be condensed in an after-condenser and,for example, passed to thermal utilization.

In this embodiment, the separation of alcohol and low boilers occurs inthe substantial absence of water. Since the alcohol and the olefingenerally have a large boiling point difference (cf. the table above),simple distillation or distillation using a small number of theoreticalplates is usually sufficient to achieve substantial removal of the lowboilers.

To avoid accumulation of low boilers in the reaction system, it has beenfound to be sufficient to treat only part of the organic phase before itis recirculated to the reaction system. In a useful embodiment, part ofthe organic phase is therefore recirculated unchanged to the reactionsystem and another part of the organic phase is treated by evaporatingand/or distilling off components having boiling points lower than thatof the alcohol and recirculating at least part of the organic phasewhich has been treated in this way to the reaction system. Preference isgiven to treating at least 20% of the total organic phase, in particularfrom 25 to 60%, e.g. from 30 to 40% of the total organic phase, obtainedin the phase separation.

The evaporation or distillation of the components having boiling pointslower than that of the alcohol can be carried out in any apparatusessuitable for this purpose, e.g. a distillation column or an evaporatorof any construction type, e.g. stirred evaporator, oblique-tubeevaporator, vertical-tube evaporator with natural or forced convection,climbing film evaporator, falling film evaporator, horizontal-tubeevaporator, Robert evaporator, Herbert evaporator, immersed-tubeevaporator, spiral evaporator, plate evaporator, Sambay evaporator orsimilar apparatuses. The bottom product from the column or theevaporator is at least partly recirculated to the reaction system.

Suitable columns are, for example, tray columns or columns comprisingordered packing or random packing elements. It is advantageous to returnpart of the overhead condensate, if appropriate after phase separationand removal of the entrained aqueous phase, as runback to the column.The other part of the overhead condensate is discharged from theprocess.

In many cases, depressurization vaporization is suitable. For thispurpose, at least part of the organic phase is depressurized into adepressurization vessel, resulting in at least part of the componentshaving boiling points lower than that of the alcohol vaporizing, and theunvaporized liquid phase is at least partly recirculated to the reactionsystem. The pressure difference in the depressurization is, for example,at least 500 mbar to a final pressure of less than 500 mbar, preferablyless than 200 mbar. If appropriate, the organic phase can be heated byindirect heat exchange before depressurization. Suitable heat exchangersare, for example, shell-and-tube heat exchangers, double-tube heatexchangers, plate heat exchangers, spiral heat exchangers, finned-tubeheat exchangers and the like. The temperature of the organic phasebefore the depressurization is selected according to the boiling pointsof the alcohol or the olefin and according to the pressure difference inthe depressurization.

The temperature is preferably sufficient for the low boilers to beessentially completely vaporized in the depressurization.

The process of the invention can in principle be applied to allesterifications in which the water of reaction is separated off bydistillation as azeotrope with an alcohol.

In the process of the invention, carboxylic acids or carboxylicanhydrides are used as acid component. In the case of polybasiccarboxylic acids, it is also possible to use partial anhydrides. It islikewise possible to use mixtures of carboxylic acids and anhydrides.

These acids can be aliphatic, including carbocyclic, heterocyclic,saturated or unsaturated, or else aromatic, including heteroaromatic.

Suitable carboxylic acids include aliphatic monocarboxylic acids havingat least 5 carbon atoms, in particular from 5 to 20 carbon atoms, e.g.n-pentanoic acid, 2-methylbutyric acid, 3-methylbutyric acid,2-methylpentanoic acid, 2-ethylbutyric acid, n-heptanoic acid,2-methylhexanoic acid, isoheptanoic acids, cyclohexanecarboxylic acid,n-octanoic acid, 2-ethylhexanoic acid, isooctanoic acids, n-nonanoicacid, 2-methyloctanoic acid, isononanoic acids, n-decanoic acid,isodecanoic acids, 2-methylundecanoic acid, isoundecanoic acid,tricyclodecanecarboxylic acid and isotridecanoic acid.

Further suitable carboxylic acid components are aliphaticC₄-C₁₀-dicarboxylic acids or anhydrides thereof, e.g. maleic acid,fumaric acid, maleic anhydride, succinic acid, succinic anhydride,adipic acid, subacic acid, trimethyladipic acid, azelaic acid,decanedioic acid, dodecanedioic acid, brassylic acid. Examples ofcarbocyclic compounds are: 1,2-cyclohexanedicarboxylic acid(hexahydrophthalic acid), 1,2-cyclohexanedicarboxylic anhydride(hexahydrophthalic anhydride), cyclohexane-1,4-dicarboxylic acid,cyclohex-4-ene-1,2-dicarboxylic acid, cyclohexene-1,2-dicarboxylicanhydride, 4-methylcyclohexane-1,2-dicarboxylic acid,4-methylcyclohexane-1,2-dicarboxylic anhydride,4-methylcyclohex-4-ene-1,2-dicarboxylic acid,4-methylcyclohex-4-ene-1,2-dicarboxylic anhydride.

Examples of suitable aromatic dicarboxylic acids or anhydrides thereofare: phthalic acid, phthalic anhydride, isophthalic acid, terephthalicacid or naphthalenedicarboxylic acids and anhydrides thereof.

Examples of suitable aromatic tricarboxylic acids (or anhydrides) aretrimellitic acid, trimellitic anhydride or trimesic acid; an example ofa suitable aromatic tetracarboxylic acid or anhydride thereof ispyromellitic acid and pyromellitic anhydride.

Particular preference is given to using phthalic anhydride as carboxylicacid component in the process of the invention.

Preference is given to using branched or linear aliphatic alcoholshaving from 4 to 13 carbon atoms, in particular from 9 to 13 carbonatoms, in the process of the invention. The alcohols are monohydric andcan be secondary or primary.

The alcohols used can originate from various sources. Suitable startingmaterials are, for example, fatty alcohols, alcohols from the Alfolprocess or alcohols or alcohol mixtures obtained by hydrogenation ofsaturated or unsaturated aldehydes, in particular ones whose synthesisincludes a hydroformylation step.

Alcohols which are used in the process of the invention, are, forexample, n-butanol, isobutanol, n-octan-1-ol, n-octan-2-ol,2-ethylhexanol, nonanols, decyl alcohols or tridecanols prepared byhydroformylation or aldol condensation and subsequent hydrogenation. Thealcohols can be used as pure compounds, as a mixture of isomericcompounds or as a mixture of compounds having different numbers ofcarbon atoms. A preferred example of such an alcohol mixture is aC₈/C₁₁-alcohol mixture.

Preferred feed alcohols are mixtures of isomeric octanols, nonanols ortridecanols, with the latter being able to be obtained from thecorresponding butene oligomers, in particular oligomers of linearbutenes, by hydroformylation and subsequent hydrogenation. Thepreparation of the butene oligomers can in principle be carried out bythree methods. Acid-catalyzed oligomerization, in which, for example,zeolites or phosphoric acid on supports are used industrially, gives themost branched oligomers. For example, the use of linear butenes gives aC₈ fraction comprising essentially dimethylhexenes (WO 92/13818). Aprocess which is likewise practiced worldwide is oligomerization usingsoluble Ni complexes, known as the DIMERSOL process (B. Cornils, W. A.Herrmann, Applied Homogenous Catalysis with Organometallic Compounds,pages 261-263, Verlag Chemie 1996). In addition, oligomerization iscarried out over fixed-bed nickel catalysts, for example the OCTOLprocess (Hydrocarbon Process., Int. Ed. (1986) 65 (2. Sect. 1), pages31-33) or the process as described in WO 95/14647 or WO 01/36356.

Very particularly preferred starting materials for the esterificationaccording to the invention are mixtures of isomeric nonanols or mixturesof isomeric tridecanols prepared by oligomerization of linear butenes toC₈-olefins and C₁₂-olefins by the octol process or as described in WO95/14647, with subsequent hydroformylation and hydrogenation.

Further suitable alkyls are alkylene glycol monoethers, in particularethylene glycol monoethers such as ethylene glycol monomethyl ether,ethylene glycol monoethyl ether and ethylene glycol monobutyl ether; andpolyalkylene glycol monoethers, in particular polyethylene glycolmonoethers such as polyethylene glycol monomethyl ether.

Particularly preferred alcohols are 2-ethylhexanol, 2-propylheptanol,isononanol isomer mixtures, decanol isomer mixtures and C₈/C₁₁-alcoholmixtures.

The esterification according to the invention can be autocatalyzed orcan be carried out in the presence of an esterification catalyst. Theesterification catalyst is appropriately selected from among Lewis acidssuch as alkoxides, carboxylates and chelate compounds of titanium,zirconium, tin, aluminum and zinc; boron trifluoride, boron trifluorideetherates; mineral acids such as sulfuric acid, phosphoric acid;sulfonic acids such as methanesulfonic acid and toluenesulfonic acid,and ionic liquids.

The esterification catalyst is appropriately selected from amongalkoxides, carboxylates and chelate compounds of titanium, zirconium,tin, aluminum and zinc. Suitable catalysts are tetraalkyl titanates suchas tetramethyl titanate, tetraethyl titanate, tetra-n-propyl titanate,tetraisopropyl titanate, tetra-n-butyl titanate, tetraisobutyl titanate,tetra-sec-butyl titanate, tetraoctyl titanate, tetra(2-ethylhexyl)titanate; dialkyl titanates ((RO)₂TiO₂, where R is, for example,isopropyl, n-butyl, isobutyl), e.g. isopropyl n-butyl titanate; titaniumacetylacetonate chelates such as diisopropoxybis(acetylacetonate)titanate, diisopropoxybis(ethylacetylacetonate)titanate, di-n-butylbis(acetylacetonate)titanate, di-n-butyl bis(ethylacetoacetato)titanate,triisopropoxy bis(acetylacetonate)titanate; zirconium tetraalkoxidessuch as zirconium tetraethoxide, zirconium tetrabutoxide, zirconiumtetrabutyrate, zirconium tetrapropoxide, zirconium carboxylates such aszirconium diacetate; zirconium acetylacetonate chelates such aszirconium tetra(acetylacetonate), tributoxyzirconium acetylacetonate,dibutoxyzirconium bisacetylacetonate; aluminum trisalkoxides such asaluminum triisopropoxide, aluminum trisbutoxid; aluminum acetylacetonatechelates such as aluminum tris(acetylacetonate) and aluminumtris(ethylacetylacetonate). In particular, isopropyl n-butyl titanate,tetra(isopropyl) orthotitanate or tetra(butyl) orthotitanate are used.

Suitable ionic liquids are, for example,1-(4-sulfobutyl)-3-methylimidazolium triflate and1-ethyl-3-methylimidazolium hydrogensulfate.

The catalyst concentration depends on the type of the catalyst. In thecase of the titanium compounds which are preferably used, this is from0.005 to 1.0% by weight based on the reaction mixture, in particularfrom 0.01 to 0.3% by weight.

When the process is carried out batchwise, the starting materials andthe catalyst can be introduced into the reactor either simultaneously orin succession. The catalyst can be introduced in pure form or as asolution, preferably as a solution in one of the starting materials, atthe beginning or only after the reaction temperature has been reached.Carboxylic anhydrides frequently react autocatalytically, i.e. in theabsence of catalysts, with alcohols to form the corresponding estercarboxylic acids (half esters), for example phthalic anhydride to formthe monoester of phthalic acid. A catalyst is therefore frequentlynecessary only after the first reaction step.

In the case of a continuous process, streams of the starting materialsand of the catalyst are fed into the reactor or, when a reactor cascadeis used, into the first reactor of the cascade. The residence time inthe reactor or the individual reactors is determined by the volume ofthe reactors and the flow of the starting materials.

The alcohol to be reacted, which serves as entrainer, can be used in astoichiometric excess, preferably from 30 to 200%, particularlypreferably from 50 to 100%, of the stoichiometrically required amount.

The reaction temperatures are in the range from 160° C. and 270° C. Theoptimal temperatures depend on the starting materials, the progress ofthe reaction and the catalyst concentration. They can easily bedetermined experimentally for each individual case. Higher temperaturesincrease the reaction rates and promote secondary reactions such asolefin formation or the formation of colored by-products. To remove thewater of reaction, it is necessary for the alcohol to be able to bedistilled off from the reaction mixture. The desired temperature or thedesired temperature range can be set via the pressure in the reactor. Inthe case of low-boiling alcohols, the reaction can therefore be carriedout under superatmospheric pressure and in the case of relativelyhigh-boiling alcohols under reduced pressure. For example, the reactionof phthalic anhydride with a mixture of isomeric nonanols in thetemperature range from 170° C. to 250° C. is carried out in the pressurerange from 200 mbar to 3 bar.

All reactors of a cascade can be operated at the same temperature.However, preference is generally given to steadily increasing thetemperature from the first to last reactor of a cascade, with a reactorbeing operated at the same temperature or a higher temperature than thereactor located upstream (based on the flow direction of the reactionmixture). All reactors can advantageously be operated at essentially thesame pressure, in particular about ambient pressure.

After the reaction is complete, the reaction mixture, which comprisesessentially the desired ester and excess alcohol, further comprises notonly the catalyst and/or its reaction products but also small amounts ofester carboxylic acid(s) and/or unreacted carboxylic acid.

To work up these crude ester mixtures, the excess alcohol is removed,the acidic compounds are neutralized, the catalyst is destroyed and thesolid by-products formed are separated off. Here, the major part of theunreacted alcohol is distilled off at atmospheric pressure or underreduced pressure. The last traces of the alcohol can be removed, forexample, by steam distillation, in particular in the temperature rangefrom 120 to 225° C. under reduced pressure. The removal of the alcoholcan be carried out as first or last work-up step.

The neutralization of the acidic substances such as carboxylic acids,ester carboxylic acids or if appropriate the acidic catalysts iseffected by addition of bases, e.g. alkali metal and/or alkaline earthmetal carbonates, hydrogencarbonates or hydroxides. The neutralizingagent can be used in solid form or preferably as a solution, inparticular as an aqueous solution. Here, sodium hydroxide solutionhaving a concentration of from 1 to 30% by weight, preferably from 20 to30% by weight, is frequently used. The neutralizing agent is used in anamount corresponding to from one to four times, in particular from oneto two times, the stoichiometrically required amount determined bytitration.

The esters of polybasic carboxylic acids, for example phthalic acid,adipic acid, sebacic acid, maleic acid, and alcohols which have beenprepared in this way are used further in surface coating resins, asconstituents of paints and in particular as plasticizers for plastics.Suitable plasticizers for PVC are dioctyl phthalate, diisononylphthalate, diisodecyl phthalate and dipropylheptyl phthalate.

The invention is illustrated by the accompanying drawing and thefollowing examples.

FIG. 1 shows a plant suitable for carrying out the process of theinvention. The plant comprises a cascade of six stirred vessels 1, 2, 3,4, 5 and 6, with the outflow from the first vessel being fed to thesecond vessel, the outflow from the second vessel being fed to the thirdvessel, etc. Alcohol is fed via the alcohol manifold 10 and feed lines11, 12, 13, 14, 15 and 16 into the stirred vessels 1, 2, 3, 4, 5 and 6.Esterification catalyst is added to the first vessel 1 via line 8. Anacid component, for example phthalic anhydride (PAn), is fed via line 7into the first vessel 1.

The vapor space of the first vessel 1 communicates via line 21 with therecycle alcohol column 9, with the vapors ascending from the firstvessel 1 being taken off via line 21 and runback from the recyclealcohol column 9 likewise being conveyed via line 21 back into the firstvessel 1. The vapor offtakes 22, 23, 24, 25, 26 from the second to sixthvessels 2, 3, 4, 5, 6 are combined via the vapor collection line 30 andlikewise lead via line 21 to the recycle alcohol column 9.

The combined vapors are fed to a condenser 31, e.g. and air-cooledcondenser. The mixed-phase stream leaving the condenser 31 is separatedin the phase separator 32. The lower, aqueous phase is taken off vialine 42. The upper, organic phase is partly fed via line 33 to therecycle alcohol collection vessel 34. Another part of the organic phasefrom the phase separator 32 is heated in the optional heat exchanger 35and depressurized into the depressurization vessel 36. As a result ofthe depressurization, the organic phase separates into a vapor fractionis enriched in the low boilers and an alcohol-enriched liquid fraction.The vapor fraction can be condensed in the after-condenser 37 and passedto a use. The liquid fraction is conveyed via line 38 to the recyclealcohol collection vessel 34. Alcohol which is separated off from thecrude ester mixture during the work-up can be fed via line 39 to therecycle alcohol collection vessel 34 and thus likewise be passed toreuse. The alcohol from the recycle alcohol collection vessel 34 is fedvia line 40 into the top or the upper region of the recycle alcoholcolumn 9 where it is conveyed in countercurrent to the ascending vaporsand it goes via line 21 back into the first vessel 1.

EXAMPLES Example 1 Preparation of Diisononyl Phthalate

The continuous preparation of 2000 g/h of diisononyl phthalate (DINP)was carried out using a cascade of 4 stirred vessels. Isononanol was fedinto each reaction vessel, a total of 1380 g/h of isononanol. 0.05% byweight of isopropyl-n-butyltitanate, based on the reaction mixture, ismetered into the first reaction vessel. In addition, 708 g/h of phthalicanhydride (PAn) were introduced into the first reaction vessel. By meansof a recycle alcohol column on the first reactor, about 1330 g/h ofisononanol mixture recycle stream were fed as runback to the recyclealcohol column.

The vapors from the first reactor were taken off via the recycle alcoholcolumn whose runback was fed back into the first reactor. The offtake ofvapor from the second to fourth reactor likewise occurred via therecycle alcohol column.

The vapors from the esterification were condensed in a water condenserand the condensate was cooled to a temperature of 70° C. The organic andaqueous phases were separated at atmospheric pressure in a phaseseparator. The water was discharged from the system; part of the organicphase (300 g/h; about 95% isononanol, 4% isononene) was fed recirculatedvia an alcohol collection vessel directly into the esterification step.

149 g/h of the organic phase were heated to 100° C. by means of apreheater and fed into a single-stage flash evaporator operated at 100mbar. The vapor phase from this flash evaporator was condensed in anafter-condenser (about 48% of water, 23% of isononanol, 29% ofisononene) and discharged from the process (7.3 g/h, of which 1.7 g/hwas isononanol). The liquid phase depleted in low boilers (isononene)from the flash evaporator (141.7 g/h, 97% of isononanol) was introducedinto the alcohol collection vessel and from there recirculated to theesterification.

An alcohol loss of 0.85 g per kg of DINP, (corresponding to 0.12 mol %of yield) therefore occurred.

Comparative Example 1

The continuous preparation of DINP was carried out in a manner analogousto Example 1, but the organic phase taken off in the phase separator wasrecirculated without after-treatment to the esterification. To preventaccumulation of isononene in the isononanol recycle stream, part of theorganic phase had to be continuously discharged from the process.

Isononene contents of more than 5% by weight in the isononanol recyclestream can lead to appreciable impairment of the product quality. Tolimit the isononene content in the isononanol recycle stream to not morethan 5% by weight, 160 g/h of organic phase had to be discharged in thepreparation of 2000 g/h of DINP, corresponding to an alcohol loss of 76g per kg of DINP (9.93 mol % of yield).

Example 2 Preparation of Dipropylheptyl Phthalate (Using a Single-StageFlash Evaporator for Discharge of Low Boilers)

The continuous production of 1280 g/h of dipropylheptyl phthalate fromPAn and 2-propylheptanol (2-PH) in the presence of isopropyln-butyltitanate as catalyst was carried out using a cascade of 4 stirredvessels. The vapors from the esterification were condensed and thecondensate was cooled to a temperature of 85° C. Organic and aqueousphases were separated at atmospheric pressure in a phase separator. Thewater was discharged from the system.

Part of the organic phase was heated to 120° C. by means of a preheaterand depressurized into a depressurization vessel maintained at 80 mbar.The vapor phase formed in the depressurization was condensed (0.1% ofwater, 27.8% of 2-PH, 72.1% of decene) and discharged from the process(7.3 g/h, of which 2 g/h was 2-PH). The liquid phase depleted in lowboilers from the depressurization was introduced into an alcoholcollection vessel and from there recirculated to the esterification.

An alcohol loss of 1.6 g per kg of dipropylheptyl phthalate(corresponding to 0.23 mol % of yield) therefore occurred.

Example 3 Preparation of Dipropylheptyl Phthalate (Using a Column forthe Discharge of Low Boilers)

The preparation of 1280 g/h of dipropylheptyl phthalate from PAn and2-propylheptanol was carried out in a manner analogous to Example 2.However, part of the organic phase was heated to 130° C. by means of apreheater and fed into a column operated at 80 mbar. The vapor phasefrom this column was condensed (12.7 g/h) and separated into an organicphase and an aqueous phase in a phase separator. The aqueous phase (2.1g/h, 99.9% by weight of water, 0.1% by weight of decene) was discarded.About half of the organic phase was fed as runback to the column, whilethe other part was discharged from the process (5.2 g/h, pure decene).

The bottom product from the column was introduced into the alcoholcollection vessel and from there recirculated to the esterification(127.1 g/h, 86% of 2-PH, 14% of decene). In this way, the alcohol lossvia the discharge of low boilers was avoided completely.

1.-11. (canceled)
 12. A process for preparing carboxylic esters byreacting a carboxylic acid or a carboxylic anhydride or a mixturethereof with an alcohol in the presence of an esterification catalystselected from among alkoxides, carboxylates and chelate compounds oftitanium, zirconium, tin, aluminum and zinc, in a reaction systemcomprising one or more reactors, with water of reaction being distilledoff as alcohol-water azeotrope with the vapor, the vapor being at leastpartly condensed, the condensate being separated into an aqueous phaseand an organic phase and at least part of the organic phase beingrecirculated to the reaction system, wherein components having boilingpoints lower than that of the alcohol are at least partly removed fromthe organic phase to be recirculated, where the components have boilingpoints lower than that of the alcohol comprising olefins which arederived by elimination of water from the alcohol used.
 13. The processaccording to claim 12, wherein the vapor is incompletely condensed,components having boiling points lower than that of the alcohol aredischarged with the uncondensed vapor, the condensate is separated intoan aqueous phase and an organic phase and the organic phase is at leastpartly recirculated to the reaction system.
 14. The process according toclaim 12, wherein the vapor is at least partly condensed, the condensateis separated into an aqueous phase and an organic phase, at least partof the organic phase is treated by evaporating and/or distilling offcomponents having boiling points lower than that of the alcohol and atleast part of the organic phase which had been treated in this way isrecirculated to the reaction system.
 15. The process according to claim14, wherein part of the organic phase is recirculated unchanged to thereaction system and another part of the organic phase is treated byevaporating and/or distilling off components having boiling points lowerthan that of the alcohol and at least part of the organic phase whichhas been treated in this way is recirculated to the reaction system. 16.The process according to claim 14, wherein at least part of the organicphase is depressurized into a depressurization vessel, resulting in atleast part of the components having boiling points lower than that ofthe alcohol evaporating, and the unvaporized liquid phase is at leastpartly recirculated to the reaction system.
 17. The process according toclaim 16, wherein the organic phase is heated by indirect heat exchangebefore the depressurization.
 18. The process according to claim 12,wherein the organic phase is recirculated to the reaction system via acolumn in which the recirculated organic phase is conveyed incountercurrent to at least part of the vapor.
 19. The process accordingto claim 12, wherein the reaction system comprises a cascade of aplurality of reactors.
 20. The process according to claim 19, whereinthe organic phase is recirculated exclusively or predominantly into thefirst reactor of the cascade.
 21. The process according to claim 12,wherein the carboxylic acid is selected from among aliphaticmonocarboxylic acids having at least 5 carbon atoms, aliphaticC₄-C₁₀-dicarboxylic acids, aromatic monocarboxylic acids, aromaticdicarboxylic acids, aromatic tricarboxylic acids, aromatictetracarboxylic acids and anhydrides thereof.
 22. The process accordingto claim 12, wherein the alcohol is selected from among C₄-C₁₃-alcohols,alkylene glycol monoethers, polyalkylene glycol monoethers and mixturesthereof.